1. Field of the Invention
The present invention relates to developing devices and more specifically to a system (referred to herein as "toner supplying device") for supplying a developing agent or toner to a developing device in a color printing system, for example, the developing device of a computer or facsimile printer or the developing device of an electrophotographic color copying machine. More particularly, the invention is concerned with a toner supplying system suitable for use in a developing device which has a plurality of developing units carried by a conveyor device adapted for conveying each developing unit successively to a latent image carrying member thereby allowing a latent image thereon to be developed.
The toner supplying system of the invention can be used not only for a rotary developing device of the type having a rotary conveyor for carrying several developing units along a circular path but it can also be used for a developing device having a different type of conveyor device such as an endless belt adapted to convey the developing units along, for example, a non-circular path, such as an oval path.
It is to be noted also that the toner supplying system of this invention is useful in both dry and wet types of developing apparatus, and for the supplying of both bi-component developing agents which contain a carrier and a toner and mono-component developing agents which comprise only a magnetic toner.
Furthermore, when the toner supplying system of the present invention is used with the developing apparatus of a color electrophotographic copying machine or a color recording device, the developing apparatus may be either a full-color or a multi-color type apparatus which employs two, three or more individual colors. It is to be understood also that the invention can be carried out successfully not only with an electrophotographic sensitive device but also with various other types of latent image carrying members such as an insulating member carrying a latent image.
2. Description of the Prior Art
In recent years, there has been a rapidly increasing demand for color copying, not only in certain specialized fields but also in the clerical field in general. This in turn has given rise to a demand for color copying machines which can be handled easily by persons who do not have any specific skill in the copying machine field as well as by professionals. Under these circumstances, and in order to meet this demand, copying machines generally referred to as a "full-color electrophotographic copying machines", which make use of electrophotographic techniques, have become widely used.
The prior art full-color electrophotographic copying machines still suffer, however, from various problems or shortcomings which need to be overcome or improved. One of these problems resides in the fact that it is quite difficult to supply toners or developing agents of different colors to respective developing units of the developing apparatus. These toners or developing agents make visible the electrostatic latent image of the different colors formed on the photosensitive member of the machine.
Various developing devices proposed and used hitherto for color electrophotography may be broadly classified in either of two types. The first type, which is generally referred to as a "side-by-side" type developing device, has a plurality of developing units containing developing agents of different colors and arranged in a side-by-side fashion along the surface of the photosensitive member. This type of developing device is the most popular at the present time. In an ordinary commercially available electrophotographic copying machine, the photosensitive member has the form of a cylindrical drum; and the developing units of the developing device are disposed in side-by-side fashion circumferentially of the peripheral surface of the photosensitive drum.
The second type of developing device is generally referred to as a "rotary" type developing device in which, as disclosed in Japanese Patent laid-Open No. 131/1972 (corresponding to U.S. Pat. No. 3,709,594), and in Japanese Patent Laid-Open No. 93437/1975, a plurality of developing units are arranged around a rotary wheel which is disposed in the vicinity of the photosensitive member. The developing units disclosed in the former of those patents use liquid type developers U.S. Pat. No. 3,987,756 discloses a dry powder developing device of the rotary type wherein a plurality of developing units are supported by a supporting member; and development with a desired color is effected by rotation of the supporting member. Also, U.S. Pat. No. 4,063,724 discloses a developing device wherein a plurality of developing units are juxtaposed along a circumferential surface of a photosensitive member.
Side-by-side type developing devices require a complicated form of construction and control. More specifically, in this type of device, it is essential for the developing units other than the operating one to be kept inoperative. This requires suitable control of the rotation of magnetic brushes in respective developing units or under arrangement to keep the inoperative developing units away from the surface of the photosensitive drum.
Another problem encountered by side-by-side type developer devices is that the diameter of the photosensitive drum inevitably needs to be increased in order to allow all the developing units to be arranged around and in the vicinity of the photosensitive drum. This makes it difficult to miniaturize the electrophotographic copying machine as a whole.
Still another problem resides in the fact that a suitable means is required to compensate for the time decay of the latent image on the photosensitive surface. This time decay is different for each of the colors because the distance between the location on the photosensitive surface where the latent image is applied and the location of each of the developing units is different. Hence, the time duration until the development of the latent image in each color varys according to the color. Compensation for this is extremely difficult to carry out and makes it difficult to adequately control the developing device and the copying machine as a whole.
In contrast, the rotary type developing device in which only the developing unit of the desired color is brought to the position near the photosensitive drum, allows a reduction in the size of the photosensitive drum and, therefore, the size of the color electrophotographic copying machine as a whole. At the same time, the rotary type developing device eliminates the necessity for compensation for time decay of the image of each color.
As is known to those skilled in the art, a developing device of the type known as a "magnet brush type" developing device makes use of a bi-component developing agent which is composed of a toner and a magnetic powder serving as a carrier. This type of developing device usually has a developing chamber which contains the developing agent. There is also provided a toner chamber for storing only the toner. When the density of the toner in the developing agent contained in the developing chamber decreases below a predetermined level, additional toner is supplied from the toner chamber to the developing chamber to reinstate the desired density. In order to store a large quantity of toner, the toner chamber is preferably large in size. The use of a large toner chamber naturally increases the size and weight of the developing device as a whole. It is quite difficult and, in normal practice impossible, to drive and control such a large and heavy developing device which has a large moment of inertia.
For this reason, hitherto, the toner chamber has been designed to be small in size and a cartridge type toner charging device has been used to charge the toner chamber with the toner as desired.
The use of small toner cartridges, however, is disadvantageous in that frequent charging of toner is required, particularly when a large number of copies is to be made. This in turn requires frequent stopping of the developing device and driving of the developing unit to the toner charging position; and it involves a laborious job of inserting the toner charging device into the developing unit so as to charge the toner chamber with the toner. Consequently, the operator is burdened by troublesome work and copying efficiency is seriously impaired.
It is to be noted also that the conventional toner supplying system explained above cannot perfectly avoid external scattering of the toner from the toner supplying device.
In addition, the developing unit of the above described prior art developing machines has to be designed such as to enable the toner charging device to be detachably attached thereto. Such a design makes it difficult to provide a developing unit of hermetic construction; and it is therefore possible that the toner or the developing agent will become scattered into the copying machine during the operation of the developing device or when the developing unit is subjected to vibration or impact at the time of starting or stopping of the developing unit. The scattered toner or developing agent not only contaminates the copying machine and the operator but also impairs the quality of the copy image due to mixing of developing agents of different colors.
These problems are encountered also by developing devices designed to be supplied with the developing agent.